1. Field of the Invention
The present invention relates to a lenticular lens sheet and a rear projection screen suitable for displaying images projected thereon by an optical image source.
2. Description of the Related Art
A known rear projection television system uses an image source including three cathode-ray tubes (CRTs), i.e., a red-image CRT, a green-image CRT and a blue-image CRT, and a rear projection screen. A rear projection screen employed in such a rear projection television system is formed by combining, for example, a Fresnel lens sheet capable of collimating light rays projected thereon in substantially parallel light rays, and a lenticular lens sheet capable of diffusing light rays to form an optical image. The rear projection screen is required to diffuse light rays in a wide range and to reduce the effect of external light on an image displayed thereon.
Referring to FIG. 6 showing a lenticular lens sheet employed in a conventional rear projection screen by way of example, the lenticular lens sheet 40 has entrance lenses 42, such as lenticular lenses, capable of gathering light rays and forming an entrance surface 41, exit lenses 47 formed near the focal points of the lenticular lenses 42, respectively, and forming an exit surface 44, ridges 48 formed between the exit lenses 47, respectively, and light-absorbing films (hereinafter referred to also as xe2x80x9cblack stripesxe2x80x9d) 49 formed on the surfaces of the ridges 48, respectively. This lenticular lens sheet is capable of diffusing light and of reducing the effect of external light.
A television projector employing an image source of cell structure, such as a liquid crystal display (LCD) or a digital micromirror device (DMD), has been developed. Such a television projector also uses the foregoing lenticular lens sheet provided with the black stripes in view of enhancing its diffusing characteristic and preventing external-light reflection. It is effective in improving contrast in images displayed on the lenticular lens sheet to increase the black stripe ratio, i.e., the ratio of the area of the black stripes to the area of the entire exit surface of the lenticular lens sheet.
However, further increase in the black stripe ratio of the foregoing conventional rear projection screen is difficult, because three color images are projected by separate projectors, such as CRTs, and the angles between the respective optical axes of the projectors are increased progressively for the reduction of the overall thickness of the rear projection television system.
The foregoing lenticular lens sheet is capable of diffusing light only in horizontal directions owing to the shape of its lenses. Therefore, the lenticular lens sheet contains optical diffusing particles (diffusing material) to diffuse light in vertical planes. The optical diffusing particles diffuse image light rays projected on the lenticular lens sheet and external light fallen on the exit surface to generate stray light rays in the lenticular lens sheet. The stray light rays thus generated deteriorates contrast in images. A means for suppressing the deterioration of contrast in images tints the entire lenticular lens sheet (body tinting), the contrast improving effect of which, however, is not necessarily satisfactory, considering reduction in transmittance attributable to tinting.
Accordingly, it is an object of the present invention to provide a lenticular lens sheet capable of enhancing contrast in images without reducing the intensity of projected optical images by suppressing the reflection of external light and of reducing the overall thickness of a rear projection television system employing the lenticular lens sheet, and to provide a rear projection screen employing such a lenticular lens sheet.
According to one aspect of the present invention, a lenticular lens sheet having an entrance surface and an exit surface comprises: a base part; an entrance lens part forming the entrance surface and having an array of a plurality of convex lens elements capable of gathering light rays; and a light absorbing layer formed in light-nongathering regions in the exit surface in which light rays refracted by the convex lens elements do not gather; in which a tinted layer is formed at least in a portion of the entrance lens part near the entrance surface.
Preferably, the lenticular lens sheet further comprises an exit lens part formed on the exit surface and having an array of a plurality of lens elements formed respectively in light-gathering regions in which light rays refracted by the convex lens elements of the entrance lens part gather.
In the lenticular lens sheet, it is preferable that the lens elements of the exit lens part are either convex or concave toward the exit surface.
In the lenticular lens sheet, it is preferable that the tinted layer contains a light diffusing material.
In the lenticular lens sheet, it is preferable that the tinted layer extends along the light receiving surface of the entrance lens part.
In the lenticular lens sheet, it is preferable that the tinted layer has portions having the shape of a wedge or a flat plane, and extending from the vertices of the convex lens elements into the entrance lens part.
According to another aspect of the present invention, a rear projection screen comprises: a lenticular lens sheet having an entrance surface and an exit surface; and a Fresnel lens sheet disposed opposite to the entrance surface of the lenticular lens sheet facing an optical image source; in which the lenticular lens sheet has: a base part; an entrance lens part formed on the entrance surface and having an array of a plurality of convex lens elements capable of gathering light rays; and a light absorbing layer formed in light-nongathering regions in the exit surface in which light rays refracted by the convex lens elements do not gather; the entrance lens part being provided with a tinted layer at least in a portion thereof near the entrance surface.
In the rear projection screen, it is preferable that the lenticular lens sheet further comprises an exit lens part formed on the exit surface and having an array of a plurality of lens elements formed respectively in light-gathering regions in which light rays refracted by the convex lens elements of the entrance lens part gather.
Preferably, the rear projection screen further comprises a front plate disposed opposite to the exit surface of the lenticular lens sheet; in which the front plate has a tinted layer formed near an entrance surface thereof or an exit surface thereof, or the front plate is entirely tinted.
In the rear projection screen, it is preferable that the lenticular lens sheet has a tinted layer formed in a portion thereof near the exit surface.
Preferably, the tinted layer has a thickness not smaller than 0.05 times the pitch of the convex lens element of the entrance lens part.
Preferably, the tinted layer has a thickness not greater than half the thickness of the lenticular lens sheet.
Preferably, the tinted layer meets an inequality: t1 greater than t2, where t1 is the thickness of a portion of the tinted layer corresponding to a central portion of each convex lens element and t2 is the thickness of a portion of the same corresponding to a peripheral portion of the convex lens element as measured along a direction perpendicular to the surface of the convex lens element.
Preferably, the light diffusing material concentration C1 of the tinted layer and the light diffusing material concentration C0 of the base part meet an inequality: 0xe2x89xa6C0 less than C1.
Preferably, the tinted layer does not contain any light diffusing material, the base part is not tinted or tinted in a color density lighter than that of the tinted layer, and a light diffusing layer containing a light diffusing material is formed between the tinted layer and the base part.
The convex lens elements of the entrance lens part may be lenticular lenses of a shape having a part which makes a tangent thereto incline to the surface of the lenticular lens sheet at an angle not smaller than the critical angle.
Preferably, the exit lens part and the front plate are provided on its exit surface with at least one of an antireflection layer, a low-reflection layer, a polarizing filter layer, an antistatic layer, a glareproof layer and a hard coating layer.
Preferably, the rear projection screen has a total light transmittance in the range of 40% to 70%.